WASHINGTON (CNN) -- Radio problems aboard a plane carrying Kentucky's governor triggered an evacuation of the U.S. Capitol less than two hours before the scheduled funeral procession for former President Ronald Reagan, federal authorities said Wednesday.

The plane landed safely at Washington's Reagan National Airport, said Doug Hogan, a spokesman for Kentucky Gov. Ernie Fletcher.

FAA spokesman Greg Martin said air traffic controllers quickly determined the plane was not a security threat, but Air Force fighter jets were scrambled to intercept it.

The evacuation was ordered about 4:30 p.m., prompting police to push back crowds of mourners lining up for a chance to see Reagan's casket pass by on its way to the Capitol.

The order was lifted minutes later.

Reagan's flag-draped casket made its way down Constitution Avenue on a horse-drawn caisson Wednesday evening, as Americans lined the route by the thousands to say farewell. (Full story)

The funeral procession ended at the Capitol Rotunda, where Reagan's casket will lie in state until Friday's funeral at the National Cathedral.

The plane that triggered the alert was authorized to enter Washington's restricted airspace, but was having problems with a radio transponder that prevented air traffic controllers from tracking the aircraft, an FAA official said.

Police ordered the Capitol, its grounds and the nearby Supreme Court building cleared out during the brief alert. Supreme Court spokeswoman Kathy Arberg would not say whether any of the nine justices was inside that building at the time.

On September 11, 2001, a hijacked jetliner entered Washington airspace and crashed into the Pentagon, killing 184 people on the ground and in the aircraft.

The Department of Homeland Security has designated all the activities in California and Washington related to Reagan's memorial a special security event, which made the Secret Service the lead agency for overseeing precautions.

The Metropolitan Police Department of the District of Columbia, the Park Police, the Capitol Police and the FBI are helping secure locations and events connected to Reagan's state funeral.

Officials suggested several measures -- including sharpshooters, bomb-sniffing dogs and undercover police roaming the crowds -- will be employed at the funeral events.

One cause for concern is that this is the first state funeral in Washington in more than 30 years, with a number of world leaders attending.

Another is the warning from the Justice Department and FBI that al Qaeda would like to attack the United States in the next few months. (Full story)